The present invention relates to plastic blow molding machines.
One of the method presently used to produce plastic resin containers, such as milk bottles, is called "blow molding." That process is widely used in the dairy industry to produce one-half gallon and one-gallon bottles from high density polyethylene, a thermoplastic resin. Generally such plastic molded bottles have an integral hollow handle to permit the filled bottles to be easily handled by shoppers and consumers.
In the blow molding process the plastic material, for example, polyethylene pellets, is placed in a hopper and the hopper leads to an extrusion screw. The screw applies pressure or heat and pressure, causing the pellets to melt and flow in the form of a viscous semi-fluid mass. The semi-fluid plastic is forced to flow around a mandrel and through an extrusion die orifice which shapes it into an elongated thin-walled tube called a "parison".
The parison, having just been forced from the extruder, is hot; for example, it may be about 350.degree. F. The parison is positioned around a blow tube and within an opened two-part mold. The mold is closed and its sealing members seal the top and the bottom of the parison to form an air-tight unit. Air is then blown through the blow tube and into the interior of the parison, causing the parison to expand against the inner walls of the mold. The walls of the mold are cooled by circulating a stream of fluid using an outer jacket surrounding the mold face. The cold walls of the mold cool the resin and form the parison, for example, into a bottle. The air is then exhausted and the two mold halves are opened.
The mold is constructed in two halves which must be brought together quickly and in exact alignment in order to prevent escape of air. At the top of the mold there is most often a "neck ring". The mold will form the body of the bottle and "flashing", usually at the top and bottom of the bottle. Since the opposite walls of the parison are hot and collapse together under pressure, they may coalesce to form the sheet-like flashing. The flashing consists of the collapsed opposite walls of the parison tube which have been brought together and which must subsequently be trimmed off from the bottle.
After the bottle is formed, the two mold halves are opened and the bottom flashing may be used to pull the bottle out of the mold and remove the formed bottle from around the blow pipe. The bottles may be removed by machine fingers (metal snap members) which reach up and grab the bottom flashing (tail). The fingers pull the bottle down onto a conveyor belt which takes the bottles to subsequent machines in which the flashing is removed.
In a typical blow molding machine, as a timing method, the extrusion screw at its forward point hits a limit switch. That switch causes a compressed air blast of low air pressure (for example, 45 pounds) for 2/10 to 3/10 of a second through the blow pin in order to slightly blow (expand) the parison before the mold is closed. This is called a "pre-blow." For example, the parison for a 6-inch diameter bottle may be expanded to 3 inches. The mold is then closed and sealed and a blast of high pressure air (for example, 80 pounds) is expelled through the blow pin to expand the parison to the interior mold wall to form the bottle. The formed bottle is cooled by the cold mold wall for 3 to 5 seconds. The blow pin is then automatically pulled upwardly, for example, for a travel of 1/2 inch, to shear the bottle neck and form a round hole.
An example of a blow molding machine is the "UNILOY" (TM) Model 350R2, which is made by Hoover Ball and Bearing Company and may be equipped with four, six, eight, ten or twelve heads (molds). When equipped with six heads to make milk and juice gallon containers (less than 65 gram gallon size) it may make 2400 or more containers an hour.
From time to time there have been certain problems with this type of blow molding machine when a "pre-finish" neck is utilized, i.e., a neck formed by a shearing action in the mold. When the mold parts are closed, a sealing and shearing member is moved into a cylindrical hollow on the blow pin, leaving a gap filled by the plastic parison, to form a seal using the parison. The blow pin is the sealing neck member attached to the blow pipe and is moved axially upwards to shear the plastic of the neck by forcing it against the shearing and sealing member which remains level.
The blow pin is supported only by the blow pipe and is not supported at its bottom end. The blow pipe is a lengthy tube which carries the compressed air and is under thermal stress because it is surrounded by a fast-moving hot tube of formed plastic.
The non-centering or non-exact alignment of the blow pin may cause difficulties because the mold is closed under great pressure. If the blow pin has been misaligned, the sealing and shearing member, called shear steels or shear rings, which meet the blow pin, may strike it with great force and may damage the blow pin. More frequently, such contact may damage or cause undue wear or stress on the shear steels. On the other hand, in order to improve the air blow pattern, users will sometimes deliberately misalign the blow pin and/or molds, which results in the blow pin being struck on each mold closure by the shear steels.
The plastic parison acts as a lubricant and prevents wear which might otherwise occur between the metal members. However, in the event of non-exact alignment between the shearing member and the blow pin, the plastic may become too thin, causing holes. If there is a hole, it will permit escape of the compressed air upon the compressed air stroke, resulting in a deformed or non-formed bottle.
There is presently being sold a "UNILOY" (TM of Hoover Ball and Bearing Company) blow pin called "Ring Of Air" (TM) in which air is expelled from an open channel, i.e., a "ring" opening, which is formed between a bottom outwardly extending flange of an inner cylindrical sleeve member and the skirt (bottom of hollow cylindrical portion) of the outer cylindrical wall portion, below the shearing-cutting edge. The parts of the blow pin are held together by a top split ring and a bottom nut. However, under the heavy and repeated striking forces of mold closing, when the shear steels hit the blow pin the nut may become loosened and the blow pin may thereby become out of alignment or even fall apart. The "UNILOY" (TM) blow pin sometimes produces faulty plastic containers in which the handle is not formed, or the handle has a "web", an undesired plastic piece in the handle. The handle webbing may partially or entirely close the handle, making the bottle difficult to fill or resulting in a partially unfilled bottle because there is no fluid (milk, juice, etc.) in the handle.
The "pre-finish" neck type of flow molding machine is generally described in U.S. Pat. No. 3,369,272; and U.S. Pat. No. 3,470,582 describes a somewhat similar machine. Both of these patents are assigned on their faces to Hoover Ball and Bearing Company, who manufactures the "UNILOY" (TM) blow molding machine. In U.S. Pat. No. 3,369,272 an "annular shearing edge 70" of "portions 68 of the mold sections 60" is used for sealing and shearing the neck of the formed plastic resin bottle; and that process is illustrated in its FIGS. 2 and 3. That patent describes the blow pin as having a head with a stepped construction. In U.S. Pat. No. 3,470,582 the "mold cavity neck portion 68" is used for separation of the parison.
In U.S. Pat. No. 3,869,237 an alternative apparatus to a movable blow pin is used to cut off the container neck, this patent showing a trim knife for that purpose.